ИСТИНА

Eunicida is a group of errant annelids characterized by their complex jaw apparatuses. It includes seven extant families and even more extinct Paleozoic groups known mostly by scolecodonts (parts of their jaws). Studying the morphology of the jaws of modern eunicids is important for the correct interpretation of paleontological findings and, therefore, for understanding the evolutionary pathways of the group. Despite the fact that data on general jaw morphology is abundant, data on jaw ultrastructure is scarce and absent for some families. The purpose of our work is to study the jaw ultrastructure of the representatives of the family Lumbrineridae. In this study, we used Scoletoma fragilis (O.F. Müller, 1776), a lumbrinerid species from the White Sea, to examine the fine morphology of its jaws in worms of various sizes and ages (from juveniles to adults). Our data reveals two principal patterns of adult worms’ maxillae structure in different parts of the jaw. The thick and mineralized plates consist of three ultrastructural layers that correlate with maxillae organization in Onuphidae and Eunicidae studied to date. However, elastic parts of maxillae have a different structure, which consists of sclerotized areas alternating with areas of collagen grid and microvilli between the sclerotization sites. The maxillae of juvenile worms differ from those of adults and consist of a single homogenous electron-dense layer. This structure resembles the jaw structure found in the Dorvilleidae, Histriobdellidae and in juveniles from the Onuphidae as well. The presence of elastic non-mineralized zones in the maxillary plates of S. fragilis is the first finding of this kind in Symmetrognatha and Labidognatha maxillae and it may be the basis for the hypothesis of continued growth of maxillae in Lumbrineridae, however, a number of unclear questions remain. The work was supported by the Ministry of Science and Higher Education of the Russian Federation under grant No. 21-14-00042.